Warn, don't assert, when storing multiple values for same key in PDF store.
Warn, don't assert, when storing multiple values for same key in PDF store.

It brings release and debug builds into parity, both will warn, neither
will abort. It's the best we can do until MuPDF can consistently handle
repairs interrupting parsing and having both objects from pre and post
repair phases around at the same time.

This fixes oss-fuzz issue 48451.

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